Since cationic electrodeposition paints have excellent coating workability and good anticorrosion properties of a coating film, they are used widely as a primer for electroconductive metal products such as automobile bodies, for which these performances are required. Recently, however, strength up of automobile bodies is devised from the viewpoint of improving safety against collision, and, since a reinforcing material is furthermore added to a member welded by spot welding, objects to be coated for a structure having a complex gap part are frequently utilized (for example, see FIG. 1). Since such structure has a gap part (hereinafter, it may be called “clearance”) of from 50 μm to 600 μm and a current density (mA/cm2) lowers in electrodeposition coating, a coating film tends to precipitate hardly and to lead to non-coating, and anticorrosion properties may be lowered.
Therefore, coating conditions are devised for securing the film thickness (μm) in the gap part, but, if just performing coating while raising the coating voltage in electrodeposition, an opening 5 of a gap structure shown in FIG. 2 is covered and a throwing property 6 in clearance coating properties cannot be obtained.
Further, when coating is performed while raising the coating voltage, there are such problems that outer panel finish properties deteriorate and outer panel thickness (μm) of an object to be coated becomes thicker to increase the use amount of the coating. Therefore, a cationic electrodeposition paint having good throwing properties (hereinafter, it may be called clearance coating properties) in an object to be coated having a gap part is wanted.
Conventionally, there is disclosed a method for forming a coating film in a gap part, characterized in that, in coating of a cationic electrodeposition paint, the maximum value (I) of a current density is expressed within 5 seconds from the start of power supply, and that a time period having a current density (0.5 I) being ½ or more of the maximum value (I) of the current density is within 5 seconds (see Japanese Patent Application Laid-Open Publication No. 2003-82498). According to the electrodeposition paint composition described in these methods for forming a coating film, however, although clearance coating properties are improved by raising a resistance value of a coating film, finishing properties deteriorate. Furthermore, anticorrosion properties of an end face are insufficient. From such a background, a paint composition that satisfies clearance coating properties, finishing properties and anticorrosion properties of an end face is wanted.